The development and practical realization of electro-optical communication systems employing fiber optic cables has witnessed the application of such systems to a variety of schemes, for example in telecommunications and computer networks, previously involving radio or copper wire cables. In order to provide full duplex communication capability, the optical fiber networks that have been employed to date have often included dedicated fibers for each direction of transmission. This approach obviously entails substantial cost and hardware, due to the number of fibers required.
One approach to alleviate this cable complexity problem is to employ a single optical fiber which carries communications in both directions. In implementing such a system, in order to provide the required channel separation, wavelength division multiplexed (WDM) communication schemes have been proposed. Examples of WDM devices are described in the U.S. patents to Nosu et al U.S. Pat. No. 4,244,045, Kobayashi U.S. Pat. No. 4,198,117, Tomlinson U.S. Pat. No. 4,111,524 and Sugimoto et al U.S. Pat. No. 4,289,373. In general, a WDM device is capable of coupling into or splitting out of two or more wavelengths of optical information. Typically the network is comprised of a pair of terminal stations interlinked by one or more repeaters distributed along the fiber optic through which the terminal stations communicate with each other. In the terminal station at one end of the single fiber highway there is usually provided an optical source or transmitter which generates a prescribed optical wavelength that is different from the optical wavelength generated by the source or transmitter at the opposite end of the highway. Also, each terminal station that contains the transmitter includes a receiver that detects the incoming wavelength transmitted by the other source. Namely, each wavelength is associated with a particular direction of transmission, one wavelength being used for transmissions in a west-to-east direction and the second wavelength being used for transmissions in the opposite or east-to-west direction over the same single fiber. To maximize isolation between channels (or wavelengths) at each terminal station, separate passband filters may also be coupled between a respective source or a detector and the optical waveguide coupling to the single fiber. An example of this type of system is described in U.S. patent to Sugimoto et al U.S. Pat. No. 4,289,373 referenced above.
Unfortunately, in this customary approach for implementing a WDM communication system, the configuration of each respective terminal station is unique, requiring that the optical wavelength to be used in a particular communication direction be dedicated beforehand, thereby preventing modification of the system once installed.